I just replaced both my front wheel bearings at 50k. I searched and couldn't find a thread on diagnosing wheel bearing failure, and the failure mode on 2nd Gen's is a little unique.
Symptoms of front wheel bearing failure:
1) Howl. This comes on gradually, making it hard to detect if you're running more aggressive tires. I've had worn tires sound worse than my wheel bearing just did.
2) Vibration - initially I felt this in the gas pedal and floorboard, but it was subtle at first. Eventually I could feel it in the steering wheel. Some people report that a long turn at speed increases or decreases the vibration, indicating failure. I didn't get that symptom.
1) Check for play in the bearing. To check this, get the front wheel off the ground, grab the tire at 12 and 6, and with force try to rock the tire. If you can, bad bearing. If you can't, try putting a pry bar under the tire and rocking the tire that way. The odd thing about 2nd gen's is that I couldn't feel any play even using a pry bar. This might be due to the more modern non adjustable style of bearing used in 2nd gens.
2) Spin the wheel by hand and listen for clicking noises. Also compare how easy it is to spin one front wheel to the other front wheel. Sometimes this will work, but I couldn't get much out of this step. The wheel has a lot of inertia once you get it going, and the brake is still dragging and making noise. So the symptoms get masked.
3) (Thanks to BamaToy for this tip.) Pull the wheel and the brake caliper. Using your fingers, grab a stud and spin the hub. It should spin easily, and fairly quickly, with almost no noise. Mine was difficult to move, and would stop as soon as I stopped spinning it. Also, it made some obvious clicking sounds.
My truck has had 33's (or perhaps bigger from prior owner) for almost all the 50k miles on the truck. IMHO, Tacoma's don't tolerate larger wheels very well. The seals on my truck were in pretty good shape and the area inside the spindle/hub that the seal protects was pretty clean, so I don't think contamination caused the failure. I'll still keep my 33's, because they're not very old, but if I were buying tires I would be going the tall/skinny route just to keep the weight down, and to minimize leverage on the bearing.